Application of Alkali Metal and Zinc Complexes in the Ring-Opening Polymerization of Cyclic Esters

Author:Cui Ya Qin

Supervisor:zuo jin cai


Degree Year:2019





From the perspective of green economy,biodegradable polyesters derived from renewable resources are very attractive substitutes for petroleum-based plastics.Among the many synthetic biodegradable polyesters,poly(α-hydroxy acid)(PAHA)represented by polylactic acid has attracted much attention due to its wide range of sources and excellent biocompatibility.Increasing the thermodynamic properties of polyester has attracted the interest of extensive researchers.The tacticity of the polymer is one main factor affecting the thermodynamic properties of polyester.Ring-opening polymerization of lactide(LA)is the main method of synthesizing PLA.The catalyst is the dominant factor affecting the degree of polymer regularity.The development of catalysts for stereoselective ring opening polymerization of lactide is important.Highly stereoselective catalysts have been reported,such as aluminum,rare earth metals,and zinc complexes,as well as monophenolic alkali metal crown ether complexes reported by our group.Alkali metal complexes can be used for the ROP of rac-LA because of their non-toxic and low-cost advantages,which not only reduces the cost of PLA,but also broadens the application range of PLA.Although our monophenolic sodium/potassium crown ether complexes had good catalytic properties,the reaction mechanism of the catalytic process has not been studied in depth.The study of the reaction mechanism will reveal the nature of the polymerization and provide direction for the design and development of the catalysts.Another important factor affecting the thermodynamics of polymers is the side chain structure.It is an important method to change the monomers structures for synthesizing polymers with stable thermodynamic properties.As analogues for LA,OCAs are rich in side groups and have higher polymerization activity.But the side reaction of racemization of monomer is a problem,especially for ManOCA,which results in a difficulty to obtain polymer with regular tacticity.Catalysts for stereoselective ROP of rac-OCAs are rarely reported until now.Based on the above reasons,the main contents of the thesis are:1:Summarization the sodium and potassium catalysts in ROP of LA and overview of the research progress in ROP of OCAs to obtain PAHA with different side groups.2:Design and synthesization of chiral potassium metal crown ether complexes and application of them in the ROP of LA.The experimental results verified that the monophenolate potassium crown ether catalysts initiated the ROP of LA by ligand-assisted monomer activated mechanism,and proved that they regulated the stereoscopic of the polymer through the chain end control mechanism further.3:Based on the above study,a series of Salen sodium ion pairs supported by multidentate and planar Salen ligands were synthesized to replace monophenols and crown ethers,and they were used for the ROP of rac-LA.The catalytic results showed that these catalysts could regulate the ROP of rac-LA stereoselectively at low temperature without crown ether,and the P_m can reach 0.82.By lowering the temperature and adding multiple amounts of the initiator,side reactions such as deprotonation of the lactide,catalyst ligand initiation,and transesterification in the ROP of LA were inhibited.DFT calculation verified the ligand-assisted activated monomer mechanism.4:A zinc complex with severely steric hindrance was synthesized and used in the ring-opening polymerization of LacOCA,PheOCA and Tyr(Bn)OCA.The racemization in the ROP of the monomer was overcome,and the stereoselective ROP of rac-LacOCA,rac-PheOCA and rac-Tyr(Bn)OCA were achieved,in-70℃their P_m are 0.97,0.96 and 0.95,respectively.5:A series of bisphenolate zinc complexes were synthesized as Lewis acid,which overcame the racemization reaction of L-ManOCA in ring opening polymerization under the assistance of pyridine,and achieved the controllable ring opening polymerization of L-ManOCA.6:Summarized all the work,and proposed the future development direction for the research topic of the thesis.